1. Field of the Invention
The present invention relates generally to a motor for generating mechanical power without using electromagnetic force and, more particularly, to a vibration driven motor.
2. Related Background Art
Conventional types of vibration driven motors are disclosed in, for example, Japanese Patent Laid-Open Nos. 62-141980, 63-214381, and 59-96881.
As shown in FIG. 11, such a motor includes a vibration member 100 made from a metallic round bar of horn shape whose root portion gradually decreases in outer diameter, a pressure member 101 made from an annular metallic member having the same outer diameter as the large-diameter portion of the vibration member 100, and two annular electrostrictive plates 102 and 103 disposed between the vibration member 100 and the pressure member 101. The pressure member 101 is fixed to the vibration member 100 by a bolt 104, and the electrostrictive plates 102 and 103 are pressed against each other. Each of the electrostrictive plates 102 and 103 has two electrodes symmetrically divided on one face, and a common electrode formed on the other face of each of the electrostrictive plates 102 and 103. The polarization direction of the two divided electrodes of the electrostrictive plate 102 and 103 differ from each other. Electrode plate 106 is disposed between the electrostrictive plates 102 and 103. The electrode plate 106 is disposed in contact with the divided electrodes of the electrostrictive plate 102 positioned on the front side, while the electrode plate 105 is disposed in contact with the common electrode of the electrostrictive plate 103 positioned on the rear side. The common electrode of first plate 102 is disposed in contact with vibration member 100.
When AC voltages, which are equal in amplitude and frequency, are respectively applied across the first plate 102 and the second plate 103 with a phase difference in time, vibration in which the vibration of the electrostrictive plate 102 is combined with that of the electrostrictive plate 103 is generated in the vibration member. The combined vibration cause a rotating motion at the tip of the vibration member moving forwardly and backwardly in the longitudinal direction of the vibration member.
FIG. 12 shows a conventional vibration driven motor which utilizes such a vibration member as a driving source. The extending end of the vibration member is pressed in contact with one face of a disk 108 to frictionally drive the disk 108 by the circular motion of the extending end of the vibration member. Thus, rotational torque is transmitted from a rotary shaft 109 fixed to the center of the disk 108.
In the above-described conventional arrangement, the vibration member does not vibrate at its extending end only, but the entire vibration assembly constitutes one vibration system. It is, therefore, necessary to support the vibration member while minimizing the influence of vibration on the same.
The vibration driven motor utilizing such a vibration member has another difficulty. For example, although the motor makes use of the motion of the extending end of the vibration member, it has been found from the experiments conducted by the present inventor that the rotational torque of the extending end of the vibration member is weak and sufficient driving torque cannot be applied to the disk which is a driven member.